This invention relates to the production of uranium and more particularly to the recovery of uranium from carbonate lixiviants through the use of precipitation inducing ion exchange resins.
Uranium is produced from uranium-bearing ores by various procedures which employ a carbonate or acid lixiviant to leach the uranium from its accompanying gangue material. The acid lixiviants usually are formulated with sulfuric acid which solubilizes hexavalant uranium as complex uranyl sulfate anions. The sulfuric acid normally is used in a concentration to maintain a pH between about 0.5 to 2.0. Carbonate lixiviants contain carbonate or bicarbonate ions or mixtures thereof which function to complex the uranium in the form of water-soluble uranyl carbonate ions. The carbonate lixiviants may be formulated by the addition of alkali metal carbonates and/or bicarbonates or by the addition of carbon dioxide either alone or with an alkaline agent such as ammonia or sodium hydroxide in order to control the pH. The pH of the carbonate lixiviants may range from about 5 to 10. The carbonate lixiviants may also contain a sulfate leaching agent such as disclosed for example in U.S. patent application Ser. No. 934,933 filed Aug. 18, 1978 by Habib. The lixiviant also contains a suitable oxidizing agent such as oxygen or hydrogen peroxide.
The leaching operation may be carried out in conjunction with surface milling operations in which uranium ore obtained by mining is crushed and blended prior to leaching, heap leaching of ore piles at the surface of the earth, or in-situ leaching in which the lixiviant is introduced into a subterranean ore deposit and then withdrawn to the surface. Regardless of the leaching operation employed, the pregnant lixiviant is then treated in order to recover the uranium therefrom. One conventional uranium recovery process involves passing the pregnant lixiviant over an anionic ion exchange resin and then eluting the resin with a suitable eluant to desorb the uranium from the resin. The resulting eluate is then treated to precipitate uranium therefrom to produce the familiar "yellowcake."
The anionic ion exchange resins employed for uranium concentration are characterized by fixed cationic adsorption sites in which the mobile anion, typically chloride, hydroxide, carbonate and bicarbonate, is exchanged by the uranyl complex anion. Such anionic ion exchange resins are disclosed, for example, in Merrit, R. C., THE EXTRACTIVE METALLURGY OF URANIUM, Colorado School of Mines Research Institute, 1971, pp. 138-147. Suitable anionic ion exchange resins may take the form of polymers or copolymers of styrene substituted with quaternary ammonium groups or polymers or copolymers of pyridine which are quaternized in form pyridinium groups.
The adsorption of uranium from aqueous solutions is described by Merrit at pages 147-156 where it is recognized that the presence of inorganic salts in the pregnant lixiviant tends to reduce adsorption of uranium by the anionic ion exchange resin. Thus, Merritt discloses at pages 147, 148, and 152 that high chloride ion concentrations tend to reduce the adsorption of uranium by the resin which results in decreased resin loading.